Just eight months after launching its first commercial drug, Somaxon Pharmaceuticals is facing a barrage from generic drugmakers hoping to capitalize on the San Diego company’s success.

Four companies have asked the Food and Drug Administration for permission to sell lower-cost generic versions of Somaxon’s Silenor, a sleeping pill designed to help people stay asleep throughout the night in addition to making them fall asleep immediately.

They based their requests on challenges of nine patents that should protect Silenor’s place in the market through 2027.

It might seem surprising that Somaxon’s nascent business came under threat so quickly after it spent more than $135 million and a decade developing its first prescription therapy and winning the right from the Food and Drug Administration to sell the pill just over a year ago.

But such challenges are increasingly common, driven by a 1984 federal law designed to get more generic drugs into the hands of consumers while providing a fair profit opportunity for the brand-name drugmakers that carry the risk of developing the original therapies.

The stakes are high for companies such as Somaxon. Generic drugmakers, on average, capture 80 percent of a brand-name drug’s market share within six months of their products going on sale, according to an April report by the IMS Institute for Healthcare Informatics.

In 2010, generic drugs accounted for 78 percent of all filled prescriptions, up from 63 percent in 2006, IMS said.

The patent challenges also increase the uncertainty facing investors who sink huge amounts of money into a drug development process that already is fraught with risks that the therapy may falter in clinical trials, get turned down by the FDA or fail to attract the interest of doctors and patients.

That could have a chilling affect on fundraising by drug developers, said Duane Roth, chief executive of Connect San Diego, a nonprofit support group for fledgling technology companies.

“Though it’s good for (driving down drug) pricing, you’re not likely to see investors invest in high-risk, long-term innovators,” he said.

Most of drug patent challenges end up in federal court, where they can be litigated for years and end in out-of-court settlements.

Critics of the secret deals say they often provide little, if any, benefit to consumers. So-called pay-for-delay agreements — when brand drugmakers pay generic makers to drop patent challenges — were the target of a congressional hearing last year.

“Such settlements effectively buy more protection from competition … and they do so at the expense of consumers, whose access to lower-priced, generic drugs is delayed, sometimes for many years,” Federal Trade Commission Chairman Jon Leibowitz said in a written version of his testimony to the Senate Subcommittee on Antitrust, Competition Policy and Consumer Rights.

In an earlier report, the commission said pay-for-delay deals put off the sale of generic drugs 17 months longer than other types of court settlements.

“That really can’t possibly be good for consumers,” said Lisa Swirsky with Consumers Union, the nonprofit publisher of Consumer Reports magazine.

“Even though the cost of drugs are still going up, they aren’t going up as fast as they used to, and a huge part of that has been the shift to generics,” she said. “If we don’t have access to generics for consumers, we’re going to go right back to the period of high escalation of drug costs.”

The Obama administration tried and failed several times to get Congress to adopt an outright ban on the settlements or to create federal oversight.

Generic drugmakers and others who oppose regulation of the deals say they help to motivate lower-cost challenges to brand drugs and offer a speedier resolution to the disputes.

“They stop a potentially lengthy and uncertain litigation process from having to play out,” said David Belian, a spokesman for the Generic Pharmaceutical Association in Washington.

Few of the battles have unfolded in San Diego County, largely because most of the drugs that come out of the local biotechnology hub are proteins and other therapeutic biological molecules, which can be difficult to copy and enjoy a stronger layer of regulatory market protection in the United States.

That could change in the coming years.

The 2010 federal health care reform law included provisions for the FDA to create a process to approve generic versions of the more complicated biological medications, also known as biosimilars. The agency is working on the new rules.

Drugs made from chemical compounds, such as Silenor, have faced more intense generic competition since the passage of the Hatch-Waxman Act in 1984, which predated the emergence of biologics as a major new class of medicines.

Several other factors made Silenor more vulnerable to challenges.

The sleeping pill is a lower-dose version of doxepin, an anti-depression and anxiety medication that has been on the market for more than 35 years.

Reformulations of existing drugs can be harder to protect with patents, said Ken Jenkins, a San Diego attorney who specializes in pharmaceutical patent law.

“It’s easier to show innovation by coming up with new compounds,” he said.

Silenor also plays in a lucrative drug space.

Physicians in the United States wrote more than 67 million prescriptions to treat insomnia in 2009, according to an IMS report cited by Somaxon in securities regulatory filings. Those orders generated more than $2 billion in sales for drugmakers.

Makers of generic drugs “have their eye on any commercial product that is successful,” Jenkins said.

Sales of Silenor totaled $4.2 million in the first six months the drug was available to patients, according to Somaxon financial reports.

The challenges to Silenor have come from Par Pharmaceutical of Woodcliff Lake, N.J.; Zydus Pharmaceuticals of Pennington, N.J.; Mylan of Canonsburg, Pa.; and Actavis of Morristown, N.J.

Of the nine patents protecting the drug, six are held by Edward Mendell, the pill’s third-party manufacturer, and cover nonactive ingredients that help bind the medication and aid in its absorption.

Two patents related to the active ingredient are held by Neil Kavey, a Columbia University psychiatrist who initially studied doxepin’s sleep-aid properties.

Somaxon licensed all of those protections and landed its own patent for the drug in February covering the dosing of Silenor at least three hours after eating to speed drowsiness and reduce sleepiness the next day. That seemingly unique trait was discovered by the company during clinical trials several years ago.

It’s not unusual for brand-name drug companies to seek new 20-year patents to extend the market life of their therapies, especially when older patents dating back to the treatment’s discovery are nearing expiration, Jenkins said.

That appears to be Somaxon’s strategy with Silenor, which faces expiration dates for its seven oldest patents in 2013 and 2015.

The company says it is seeking several other patents that would cover additional aspects of the drug that were discovered in clinical trials, such as treating insomnia in the elderly, preventing early awakenings and aiding sleep without triggering weight gain, the company said in regulatory filings.

The four generic companies that want to sell their own versions of the drug have targeted one of Kavey’s patents for the treatment of transient and short-term insomnia that expires in 2020 and the patent recently acquired by Somaxon, which runs until 2027.

Efforts to obtain comments from the generic drug companies were unsuccessful.

Somaxon has responded to the challenges by filing lawsuits against the generic drugmakers. The court actions triggered a 30-month delay in the cheaper drugs getting to market under a provision of the Hatch-Waxman Act that gives brand-name pharmaceutical companies breathing room to settle the litigation.

“This is exactly what you see in the industry,” Jenkins said. “It’s a very good example of how this patent game works.”

Somaxon officials declined to comment.

Instead, they referred to earlier written statements in which the company vowed to “vigorously enforce its intellectual property rights relating to Silenor.”